1. Field of the Invention
The present invention generally relates to a laser beam scanning apparatus, and more particularly to a laser beam scanning apparatus for controlling a diameter of a light spot on a recording medium of an image forming system that is capable of varying the resolution of images on the recording medium.
2. Discussion of the Background
Laser beam scanning devices of image forming systems, such as digital copiers, laser printers and laser facsimile machines, are known. FIG. 1 shows a known laser beam scanning device. This laser beam scanning device comprises a light source unit 1 for emitting a collimated laser light ray, a cylinder lens 2 for converting the laser light ray from the light source unit 1 into a convergent light ray (which converges only in a sub-scanning direction), a rotary polygon mirror 3 for deflecting the convergent light ray from the cylinder lens 2, an f.THETA. lens 4 for converting the deflected light ray into a convergent light ray (which converges in a main scanning direction) and for focusing the convergent light ray on a recording surface of a photosensitive medium 5 as a light spot. As the rotary polygon mirror 3 is rotated at a constant rotating speed, as indicated by the arrow "a" in FIG. 1, the light spot scans the recording surface of the photosensitive medium 5 in a main scanning direction, as indicated by the arrow "b" in FIG. 1, to record an image thereon. The laser light ray, deflected from the rotary polygon mirror 3, is corrected by the f.THETA. lens 4 to make the light spot move on the recording surface of the photosensitive medium 4 at a constant linear speed.
The laser beam scanning device in FIG. 1 further comprises a sync photodetector 6. The sync photodetector 6 receives the laser light ray from the rotary polygon mirror 3 and detects the timing of the start of each of the main scannings on the photosensitive medium 5. The recording surface of the photosensitive medium 5 is scanned in the main scanning direction in accordance with the rotation of the rotary polygon mirror 3. The sync photodetector 6 outputs a sync signal for each of the main scannings.
FIG. 2 shows the light source unit of the laser beam scanning device in FIG. 1. In FIG. 2, the light source unit 1 comprises a laser diode (LD), a collimator lens 1b, an apertured unit 1c, an LD circuit board 1d, and a casing 1f.
In FIG. 2, the laser diode (LD) 1a is connected to an LD control circuit of the LD circuit board 1d and is supported on the LD circuit board 1d. The laser diode (LD) 1a emits laser light rays according to a light emission current supplied from the LD circuit to the laser diode (LD). The light emission current is modulated in accordance with image data. The collimator lens 1b converts the laser light rays from the LD 1a into a collimated light beam. The apertured part 1c has an opening 1e by which the cross-sectional shape of the collimated light beam from the collimator lens 1b is restricted. These parts are accommodated in the casing 1f. Thus, the light source unit 1 emits the collimated light ray toward the rotary polygon mirror 3 in FIG. 1.
FIG. 3 shows the sectional shape of the opening of the apertured part 1c. The sectional shape of the collimated light ray from the light source unit 1 is changed by the aperture part 1c into the sectional shape of the aperture opening shown in FIG. 3.
For recent digital image forming systems, the capability to vary the resolution of images on a recording medium is needed. This capability is called a multi-resolution capability. It is desirable to provide an image forming system in which one of a plurality of resolutions of images (for example, 240 dpi, 300 dpi, 400 dpi and 600 dpi) on the recording medium can be selected by the user. In order to realize this capability, it is necessary to vary the diameter of the light spot on the recording medium in accordance with the user's selection. However, in the laser beam scanning device described above, the optical parts are produced and assembled according to the design of production models. It is difficult for the user to vary the diameter of the light spot on the recording medium in accordance with the user's choice by using the above laser beam scanning device.
One of the conceivable methods of varying the diameter of the light spot on the recording medium is that the apertured part of the laser beam scanning device is replaced by another apertured part whose opening has a desired shape and size. However, it is necessary to disassemble the image forming system, and, after the parts change, the new apertured part must be accurately aligned in the optical system by adjustment. Therefore, it is very difficult for the user to replace the apertured part of the laser beam scanning device by another part.
Another method of varying the diameter of the light spot is that a plurality of apertured parts having openings with different sizes are mounted on the laser beam scanning device and a mechanical unit for selecting one of the apertured parts is provided. However, it is difficult to accurately align each of the apertured parts within the optical system by adjustment. Also, in order to mount the plurality of apertured parts, it is necessary to make the size of the light source unit much greater. The size of the laser beam scanning device also becomes greater. Also, in order to mount the additional selecting unit on the laser beam scanning device, it is necessary to make the production cost higher. Thus, the above method is not appropriate for the needs of the user.
Still another method of varying the diameter of the light spot is to attach a different aperture part having a desired-size opening to the apertured part of the laser beam scanning device. However, it is difficult for the user to accurately align the attached part within the optical system by adjustment. Also, when the above mentioned method is selected, it is necessary to form the opening of the aperture part to be attached into the accurate shape and size. If the opening of the attached part is formed inaccurately, the occurrence of diffracted light rays due to the attached part becomes another problem.
The opening of the aperture part is formed through punching or photo-etching. The opening of the apertured part shown in FIG. 3 has a 2.4 mm by 0.4 mm size. However, when the opening of the apertured part is formed through punching or photo-etching so as to have such dimensions, it is unavoidable that a burr having a size on the order of 0.01 mm to 0.05 mm is produced at an edge of the opening of the apertured unit. FIG. 4 shows a burr 1g produced at an edge of the opening of the apertured unit 1c. For example, the burr 1g at the edge of the aperture opening is approximately 0.05 mm wide and 0.03 mm high, as indicated in FIG. 4. Thus, when the above method of varying the diameter of the light spot is selected, the occurrence of a diffracted light ray due to the burr at edge portions of the opening of the attached aperture part is unavoidable. Such a diffracted light ray will result in a significant problem in the optical scanning of the laser beam scanning device described above. Therefore, the above mentioned method is also not appropriate for the needs of the user.
A different method of varying the resolution of images on the recording medium without changing the shape or size of the opening of the apertured part is also conceivable. This method is to control the laser beam write clock, on which the recording of the dots to the recording medium is based, in accordance with a desired dot density. For example, the optical scanning can be performed based on a laser beam write clock corresponding to 400 dpi while the diameter of the light spot equivalent to that of 300 dpi recording is used. However, the resolution of images produced by making use of the above mentioned method becomes approximate and not accurate. In order to accurately change the resolution of images to a desired dot density, it is necessary to change the diameter of the light spot on the recording medium to a desired diameter by using one of the light spot diameter varying methods mentioned above. However, any of the conventional methods mentioned above is not suitable for the needs of the user. It is very difficult for the user to vary the resolution of images on the recording medium to a desired image resolution on the conventional laser beam scanning device.